The coupling devices are known in various designs. A major advantage is the contactless transmission of torque via the air gap between the hysteresis part and the rotor part. Operation is based on the effect of magnetic fields building up and poles attracting each other in synchronous operation or on the continuous magnetic reversal of the hysteresis part moving past the magnets in the slip mode of operation.
DE 39 05 216 A1 describes an electrically controllable hysteresis coupling that is used to control tensile forces in winding operations. This hysteresis coupling serves to couple a drive shaft with an output shaft coaxial therewith. The drive shaft is mounted with an armature part having a hysteresis part connected therewith which corresponds to a rotor part in which magnets are arranged. The action of force of the magnets can be varied via a coil fixedly arranged in a housing, whereby the slip of the hysteresis coupling and thus the number of rotations of the output shaft can be adjusted with respect to the drive shaft. In this structure, the permanent magnets are arranged radially on either side of a hysteresis ring rotating contactlessly in an axial groove.
Such a design is disadvantageous, however, in that, even when the voltage applied to the coil is turned off, the permanent magnets still transmit a torque from the drive shaft to the output shaft. Even with a negative energizing of the coil, this moment cannot be eliminated completely.
DE 10 2004 057 848 A1 describes an adjustable hysteresis coupling which in particular serves to drive auxiliary aggregates of a vehicle. In this design, a rotor part is driven which has a gap into which a hysteresis part extends, which hysteresis part is fastened to an armature secured to an output shaft. A coil generates a magnetic field which continuously reverses the polarity of the hysteresis ring while the rotor part is driven, so that the coupling operates in the slip mode. In order to guarantee the rotation of the output shaft, for instance for driving a coolant pump, even if the current fails, an additional permanent magnet is arranged in the stator behind the coil, which also generates an electromagnetic field acting on the hysteresis part. However, this has the effect that it is not possible, for instance, in case of a cold start of an internal combustion engine, to completely stop the pump from conveying, i.e., to transmit no rotation. Besides this electromagnetic failsafe safety measure, a mechanical failsafe security measure using a friction clutch is also described.
A disadvantage of both designs described above is that either a start-up of the output shaft at the beginning of the rotor movement cannot be prevented or that a transmission of torque is not guaranteed in the event of a current failure.